As a concave-convex surface inspection apparatus, an apparatus having a slit light source emitting a slit light to an object to be measured, a camera taking a slit image of the object to be measured when the object to be measured is illuminated with the slit light, and an image processing section obtaining three dimensional data from the slit image taken by the camera is known (for example, JP H4-301707A, paragraphs 0005 and 0006 and FIG. 3). In the known device, the slit light beams are diffused in a shape of a fan relative to an optical axis of the slit light. Therefore, when inspecting a surface on which a number of relatively deep grooves are formed, blind spots are generated in grooves which receive slit light beams that are projected away from the optical axis. Consequently, the image of the surface illuminated with the slit light beams which are diffused in the shape of the fan includes the groove whose bottom portion is not completely captured in the image. As a result, a problem arises that a shape of the hidden portion of the groove is not measured.
Further, a height measuring apparatus having a light source emitting a slit light to an object to be measured from an oblique direction, image sensors taking images of the object to be measured illuminated by the reflected slit light via each of telecentric lenses, and an image processing means obtaining a height of the object to be measured based on the images of the object to be measured illuminated by the reflected slit light taken by the image sensor is known (for example, JP 2001-194116A, paragraphs 0011 and 0012 and FIG. 1). The known apparatus further includes a half mirror positioned in a direction in which the slit light is reflected from a surface of an object to be measured. The reflected image of the slit light passes through the half mirror and reflects on a surface of the half mirror in a different direction from the direction that the reflected image passes through the half mirror. Consequently, each of the image sensors is positioned so as to take the reflected image that passes through the half mirror and the reflected image that reflects on the surface of the half mirror respectively. According to the height measuring apparatus, the image sensors reliably take the reflected images of the slit light by use of the telecentric lenses. However, an optical axis of the slit light and an imaging optical axis are inclined relative to the vertical axis at 45 degrees, and thus blind spots are generated when taking an image of a surface having a number of relatively deep grooves.
A need thus exists for a concave-convex surface inspection apparatus which is not susceptible to the drawback mentioned above. A need thus exists for a concave-convex surface inspection apparatus that reduces blind spots occurring when measuring a concavo-convex surface of an object to be inspected.